Just happened to come across this article which, while not directly related to the discussion, does highlight my point on composites and one of the reasons why Rutan wouldn't be interested in an AN 225.

Purchasing and modifying an ex-Soviet carrier aircraft to become (effectively) an ICBM launch platform may well fall foul of ITAR restrictions. Rutan is already on record on complaining that ITAR creates roadblocks in working with British engineers at Virgin Galactic.

ITAR is only the exportation of US technology and intelligence to outside countries, nothing forbids the importation.

More than likely, Rutan has already scoured for potential launching platforms and found that either they don't get to the altitudes required for his mass fraction/isp requirements or their acquisition/modification/operational costs exceed starting from scratch.

The SS1 had a launch altitude of ~65,000 ft; not very many airplanes can carry to that altitude, let alone the potential size of the SS2 if it's supposed to be carrying 6+ people!